1. Field of the Invention
The present invention is directed to a method for reconstructing 3D image data for a volume of interest of an examination subject, wherein radiation emanating from a radiation source is received with a planar detector, of the type wherein a number of 2D central projections are acquired from different projection directions, a volume of interest is marked, and 3D image data of the volume of interest corresponding to the markings are reconstructed from the 2D central projections.
2. Description of the Prior Art
In the current state of computer technology, reconstructive 3D imaging represents has widespread use. This is particularly true for medical-diagnostic imaging, for example computed tomography (CT), magnetic resonance tomography, nuclear medicine, ultrasound and, recently, 3D x-ray technology. These methods are also utilized, outside medical technology in general technology such as, for example, computer tomography for the non-destructive testing of materials (motor blocks in the automotive industry, drill cores in the petroleum industry, etc.).
The term xe2x80x9creconstructive imagingxe2x80x9d means that the measured data supplied by the respective detectors are not directly interpreted, but are used as inputs in a procedure that supplies qualitatively new image information, i.e. image data. The mathematical algorithms utilized for this purpose make high demands on the computers employed for the processing of these algorithms with respect to the computing power and data volume.
An important structural feature CT apparatus is a mechanically stable, usually annular gantry that allows substantially vibration-free revolutions of the entire measuring system in the sub-second range. A disadvantage of such a CT apparatus is the limited accessibility to the patient for medical personnel, for example a physician, as a consequence of the gantry. Better accessibility would be desirable for the more frequently utilized minimally invasive and endoscopic surgical techniques (interventions) with additional 3D imaging.
One technique in this direction is the reconstruction of 3D image data from a series of 2D central projections acquired in the form of standard x-ray exposures with a C-arm apparatus that is conventional in terms of its mechanical structure, whereby a planar detector, for example an x-ray image intensifier or, recently, a semiconductor panel is utilized as the radiation receiver.
For example, neuro-radiology is field of employment for such a technique. Vessels filled with contrast agent and their spatial position are imaged with high topical resolution. This is required, for example, in the neuro-surgical treatment of aneurisms. Interventions of this type ensue under constant x-ray monitoring.
The technical realization of the 3D functionality ensues by acquiring the digital data corresponding to the 2D central projections in the course of rotation angiography. For example, a C-arm apparatus distributed by Siemens AG under the name NEUROSTAR(copyright) is suitable as a registration device. Typically, 50 2D central projections having 1024xc3x971024 pixels each are registered in five seconds over an angular range of 200xc2x0. Due to the mechanical instability of the C-arm, the exact projection geometry must be defined for each of the 2D central projections and must then be taken into consideration in the implementation of the reconstruction algorithm. The reconstruction of the 3D image data ensues according to CT principles.
A C-arm apparatus of this type is described in detail in H. Barfuss, Digitale 3D-Angiographie, VDE-Fachbericht, Vol. 34: Das Digitale Krankenhaus, VDE-Verlag, 1998.
In rotation angiography as a recent 3D imaging method, the preconditions compared to computed tomography are essentially modified by the following points:
A mechanically unstable system with a freely rotatable C-arm is utilized.
The objective is interventional employment, i.e. the image result, must be quickly available during the examination or treatment.
The xe2x80x9cfield of visionxe2x80x9d of the detector, i.e. the aperture angle of the cone-shaped or pyramidal x-ray beam emanating from the x-ray source, is limited compared to computed tomography.
The following facts follow from these points:
1. The entire body is usually not registered, but only a part thereof. This defines a maximum reconstructable volume (MRV).
2. The maximally obtainable spatial resolution of the portrayed volume is limited by the resolution of the 2D central projections; the resolution available to the observer is additionally limited by the selected size of the voxels (voxel=volume element).
3. The number of voxels enters critically into the calculation time. A halving of the size of the voxels with retention of the size of the volume to be reconstructed means, for example, an eight-times increase in the number of voxels and also means an eight-times increase in the size of the dataset. Given limited calculating time (for reconstruction and display), a larger volume with poorer spatial resolution, or a smaller volume with high-spatial resolution (limited by the resolution of the 2D projections) therefore can be reconstructed with a given calculating power.
4. During the implementation of an intervention (for example, placement of platinum coils), the physician is interested in obtaining optimally high resolution, local 3D information with respect to a volume of interest (VOI=volume of interest) within the MRV.
Given employment of rectangular surface detectors, the MRV can be considered approximately as a circular cylinder around the rotational axis of the C-arm in an approximation, as shown in FIG. 3 herein.
The selection of the volume to be reconstructed based on this approximation is described in detail below.
The definition of the volume within the MRV from which 3D image data are to be reconstructed ensues on the basis of numerical coordinates, usually in a global coordinate system that is preferably oriented with respect to the geometry of the apparatus. For example, the rotational axis of the C-arm corresponds to the z-axis, the rotational plane corresponds to the xy-plane and the x-axis proceeds parallel to the patient support.
The selected volume is geometrically considered as a cuboid, composed of many small cuboids of the same size, i.e., the voxels. The reconstruction allocates a gray scale value to each voxel, this corresponding to the x-ray attenuation coefficient (approximate density) of the subject in the region of the voxel. The reconstructed 3D image data therefore represent a scalar 3D field f (i, j, k), with
i=1, . . . , Nx,
j=1, . . . , Ny,
k=1, . . . , Nz,
where Nx, Ny, Nz reference the number of voxels which are present in the direction of the respective coordinate axis.
The mid-point of each voxel has a geometrical position (xi, yj, zk) allocated to it. When the edge lengths of a voxel are referenced dx, dy, dz, then, for example, the following applies:
xi=x0+i*dx,
yj=y0+j*dy,
zk=z0+k*dz
The reference point (x0, y0, z0) describes the hypothetical voxel that lies outside the cuboid on the spatial diagonal thereof and touches it. Of course, other reference points are possible, for example the mid-point of the cuboid (xM, yM, zM).
N=Nx*Ny*Nz is the overall number of voxels. This number N critically defines the required calculating time. The quantities X=Nx*dx, Y=Ny*dy and Z=Nz*dz describe the edge lengths of the cuboid, i.e. the illustrated overall volume. The quantities dx, dy, dz define the topical resolution of the reconstructed 3D dataset.
Given a constant N, i.e. a given calculating time, one can thus construct either a large volume with poor resolution or a small VOI with good resolution. For evaluating diagnostically or therapeutically relevant structures, for example an aneurism, the latter is preferred. A problem is having to indicate the position of the VOI in the space for a particular examination situation, for which abstract positional coordinates (reference point) are less helpful since these, unfortunately, have no direct reference to the examination subject, i.e. the patient.
To address this problem, it is known to proceed as described in J. Moret et al., 3D rotational angiography: Clinical value in endovascular treatment, Medica Mundi, Vol. 42, no. 3, 1998, oder bei R. Kemkers et al., 3D-Rotational Angiography: First clinical application with use of a standard Philips C-arm system, CAR""98, edited by H. U. Lemke et al., Elsevier Science B.V., 1998. As described therein, only after a reconstructed MRV with reduced resolution is produced are artificial orthogonal projections generated therefrom, for example parallel beam MIPs (MIP=maximum intensity projection) wherein the VOI can then be defined.
Such a procedure is complicated and time-consuming.
It is an object of the invention to provide a method of the type initially described wherein the VOI can be defined in a simple and time-saving way.
This object is inventively in a method for reconstructing 3D image data for a volume of interest of an examination subject, wherein radiation emanating from a radiation source is received with a planar detector, and wherein a number of 2D central projections are acquired from different projection directions, at least two 2D central projections from the 2D central projections are displayed, the contour of the volume of interest is marked in a first, displayed 2D central projection(s) and a corresponding marking is mixed into the first, display 2D central projection(s), a marking is mixed into the other displayed 2D central projection(s) that show the contour(s) corresponding to the mark mixed into the first 2D central projection(s), and 3D image data of the volume of interest containing the marks are reconstructed from the 2D central projection(s).
The inventive method thus allows the selection of a VOI in an easy, dependable and fast way, without the time-consuming intervening step of reconstructing the MRV in reduced resolution and without the determination of parallel beam MIPs, since the inventive procedure allows the interactive selection of a VOI directly from measured 2D central projections that must be available anyway for the reconstruction of the 3D image data. The images with reference to which the selection of the VOI ensues are conventional x-ray exposures in central-perspective imaging.
In the limit case, stereoscopic image pairs can be used in the selection of 2D central projection serving as a VOI. As a rule, the projection directions of the 2D central projections will reside substantially orthogonally relative to one another. Angles between the projection directions that lie between 0xc2x0 and 90xc2x0 or are greater then 90xc2x0 are also possible, wherebyxe2x80x94in the orthogonal casexe2x80x94a more precise position and size determination is possible. Even though two 2D central projections would suffice in most instances in order to select a VOI, the possibility also exists within the scope of the invention to display more than two 2D central projections for the selection of the VOI and to mix corresponding markings into these displays.
When employing a C-arm apparatus, the mechanical instability thereof leads to irregularly positioned exposure positions of the individual 2D central projections; however, the individual exposure positions are at least fundamentally known. They are taken into consideration not only in the reconstruction algorithm in the determination of the 3D image data but also in the selection of the VOI. An advantageous method for describing the projection geometry is the employment of homogeneous coordinates. A central projection is thereby completely described by a 3xc3x974 matrix P. When this matrix is applied to a point of the 3D space, then, after re-normalization, the 2D detector coordinates (see u and v according to FIG. 4) of the picture element are directly obtained. (See N. Navab et al., 3D Reconstruction from Projection Matrices in a C-Arm based 3D-Angiography System, Medical Image Computing and Computer-Assisted Intervention-MICCAI""98, edited by W. M. Wells et al., Springer, 1998.)
In a preferred embodiment of the invention the marks are modified as required before the reconstruction of 3D image data, whereby given modification of the marks mixed into a displayed 2D central projection, the mark(s) mixed into the other displayed projection (s) is (are) correspondingly adapted. There is thus the possibility of interactively optimizing the selection of the volume of interest in a number of steps, and modifications of the marking undertaken in one of the 2D central projections can be immediately recognized in terms of their effect in the other, displayed 2D central projections.
The time that is required for the reconstruction of the 3D image data for a volume of interest can be further reduced in a version of the invention wherein only those data subsets that are absolutely necessary for the reconstruction of 3D image data of the volume of interest corresponding to the markings are taken into consideration in the reconstruction of 3D image data for the volume of interest corresponding to the marks, namely from the data corresponding to the individual 2D central projections. This is advantageous particularly given interventional utilization of the inventive method, since it is important in such procedures to obtain a diagnostic answer as quickly as possible in the form of high-resolution 3D image data for the volume of interest corresponding to the marks.
In order to obtain further information with respect to the selected volume of interest, in an embodiment of the invention arbitrary, further 2D central projections can be selected into which marks corresponding to the contour of the volume of interest are mixed. There is also the possibility of mixing marks corresponding to the contour of the volume of interest into all 2D central projections. This affords the possibility to check the selected volume of interest, by successively viewing the 2D central projections provided with marks such as by scrolling. Alternatively, there is the possibility in another version of the invention to display all 2D central projections into which a mark is mixed in the form of a cine replay, i.e. successively in the fashion of a movie film, with mixed-in markings.
In another embodiment of the invention, substantially rectangular markings are mixed into the 2D central projections, since this can be realized with little calculating outlay. The mark can be a reticule in order to identify the center of the selected volume of interest, and the reticule lines of the reticule can be interrupted in a central region in order to prevent the diagnostically relevant structures from being covered by the reticule lines.